Robert Baratheon once spent a fortnight on Estermont with his mother's family. There he bedded a widowed cousin he used to play with as a child.[6]

Old Lord Estermont was one of the insulting matches Prince Doran Martell proposed to his daughter Princess Arianne.

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A Clash of Kings

On the outset of the War of the Five Kings, Lord Estermont declares for his grandson and liege, King Renly Baratheon. The Estermonts' sea turtle is seen by Catelyn Tully in Renly's camp. When he is preparing for battle against his brother Stannis Baratheon, Renly decides Lord Estermont would command the reserve of his army. The Estermonts turn to Stannis after Renly's death, however, along with most other storm lords.

Ser Cortnay Penrose acknowledges the presence of the great lords Estermont, Errol and Varner during his parley with Stannis. Lord Estermont recommends against storming the walls of Storm's End, believing it would cost too many lives. He suggests a duel against Cortnay, or a long siege to starve them. He refuses to turn to the religion of the Lord of Light, and asks to fight under the crowned stag banner instead of the flaming heart.

Because of contradictory information, there are different possibilities for the Estermont family tree; see House Estermont/Family Trees.
It can be said with certainty that Cassana Estermont had two brothers.

With unspecified familiar relationship to the main branch of the house there is also:

An unnamed female cousin, whose father and husband died during the Siege of Storm's End. She was of an age with her cousin Robert, with whom she would play when they were both still very young.